New California Data Reinforce the Evidence That Omicron Is Less Virulent Than Delta

Omicron patients were much less likely to have severe symptoms.


A new study of COVID-19 patients in California reinforces the evidence that the omicron variant, while highly contagious, is much less likely to cause serious symptoms than prior iterations of the coronavirus. Compared to people infected by the delta variant, the researchers found, people infected with omicron were half as likely to be hospitalized, one-quarter as likely to require intensive care, and less than one-tenth as likely to die. When omicron patients were admitted to a hospital, their average stay was 70 percent shorter.

Berkeley epidemiologist Joseph Lewnard and his collaborators examined the records of nearly 70,000 Kaiser Permanente patients in Southern California who tested positive for COVID-19 from November 30 to January 1. During that period, omicron accounted for three-quarters of COVID-19 cases. While 11 of the delta patients received mechanical ventilation, none of the omicron patients did; just one omicron patient died, compared to 14 deaths in the delta group. "During a period with mixed Delta and Omicron variant circulation," Lewnard and his co-authors report, "SARS-CoV-2 infections with presumed Omicron variant infection were associated with substantially reduced risk of severe clinical endpoints and shorter durations of hospital stay."

Recent COVID-19 trends in the United States and other countries show that cases are exploding. But deaths have risen much less dramatically, making the current surge far less lethal than last winter's. According to theĀ New York Times COVID-19 database, the seven-day average of newly identified cases has more than sextupled in the last month, while the seven-day average of daily deaths has risen by 34 percent.

One possible explanation for that contrast is increased resistance to COVID-19 as a result of vaccination and prior infection. But because the California study compares delta and omicron outcomes in the same place during the same period, its results suggest that omicron is inherently less dangerous than delta to any given patient, possibly because it is less apt to infect the lungs.

Even among unvaccinated patients, those infected by omicron were 60 percent less likely than those infected by delta to be hospitalized with symptoms. The reduced risk from omicron also held up across age groups, and it was apparent regardless of whether patients had comorbidities or had previously been infected by COVID-19.

Lewnard et al. note that the "consistency of the association of Omicron variant infection with reduced risk of hospitalization across age and comorbidity categories, and regardless of prior immunity from vaccination or SARS-CoV-2 infection, during the same month and in the same population, argues against host or behavioral factors as causes of the observed disease attenuation with the Omicron variant." They add that "the finding of a reduction in severity of Omicron in patients with known prior infection is compelling evidence of an intrinsically less severe infection, rather than only different (more immune) persons becoming infected with the Omicron variant."

Previous research also has found that omicron infection is associated with a reduced risk of severe symptoms. Between October 1 and December 6, according to a South African study, COVID-19 patients infected by omicron were 80 percent less likely to be hospitalized than patients infected by other variants. A Scottish study that looked at patients who tested positive for the coronavirus from November 1 to December 19 found that "Omicron is associated with a two-thirds reduction in the risk of COVID-19 hospitalisation when compared to Delta."

According to an English study of people tested from December 1 through December 14, omicron patients were 41 percent less likely than delta patients to stay in the hospital overnight. A Canadian study of cases identified between November 22 and December 25 found that omicron patients were 65 percent less likely to be hospitalized than delta patients.

"These findings collectively suggest that differences in viral factors between the Omicron and the Delta variants, such as differences in viral tropism or virulence factors, might be driving the observed relative reductions in disease severity," Lewnard et al. write. "Recent ex vivo studies demonstrate higher replication of the Omicron variant in the human upper respiratory tract as compared to the small airways of the lung, consistent with animal experiments suggesting that disease from infection with the Omicron variant might be confined to the large airway."

Like other studies, Lewnard et al. say, "our findings suggest vaccine protection against infection with the Omicron variant may be lower than protection against infection with the Delta variant." But also consistent with other data, this study indicates that vaccination provides strong protection against severe disease caused by either variant.

Among cases identified in outpatient settings, for example, delta patients who had received three vaccine doses were 85 percent less likely to be hospitalized with symptoms than unvaccinated delta patients; the risk reduction for omicron patients was 62 percent. The authors say "evidence for a reduction in severe outcomes among vaccinated cases with both Delta and Omicron variant infections in our study…suggests substantial public health benefits from continued COVID-19 vaccination."